Electronic devices need an electrode that provides a low contact resistance with a semiconductive material. Example of electronic devices include organic and inorganic electronic devices. Examples of organic electronic devices include organic light emitting diodes (“OLEDs”) (the OLED can be used in, for example, a display or as a light source element of a light source used for general purpose lighting), organic solar cells, organic transistors, organic detectors, and organic lasers. Such devices typically include a pair of electrodes (e.g., an anode and a cathode) with at least one semiconductive layer between the electrodes.
In the particular case of the OLED, the OLED is typically comprised of two or more thin organic layers (e.g., a conducting polymer layer and an emissive polymer layer where the emissive polymer layer emits light) separating its anode and cathode. Under an applied potential, the anode injects holes into the conducting polymer layer, while the cathode injects electrons into the emissive polymer layer. The injected holes and electrons each migrate toward the oppositely charged electrode and produce an electroluminescent emission upon recombination in the emissive polymer layer.
The OLED's cathode is typically a multilayer structure that includes, generally, a thin electron injecting layer that has a low work function, and also a thick conductive layer such as, for example, aluminum, silver, magnesium, gold, copper, or a mixture thereof. The electron injecting layer with the low work function provides an electrically conductive path for current flow as well as a way to efficiently inject electrons into the adjacent emissive polymer layer. One problem with low work function metals is that they readily react with the environment (e.g., oxygen and moisture). For example, a low work function calcium cathode survives only a short time in air due to rapid device degradation caused by atmospheric moisture and oxygen. It would be desirable to have an electron injecting layer that has a low work function and is less likely to react with the environment.
In addition, the electron injecting layer can be difficult to deposit on the semiconductive layer of the electronic device because, for example, some materials have a high melting point. It would be desirable to have an electron injecting layer that can be easily deposited on the semiconductive layer.
In the OLED, typically, different electron injecting materials are used depending on the type of emissive polymer layer on which it is placed (different types of emissive polymer layers are used depending on, for example, the desired color that is to be emitted by the layer). For example, a first electron injecting layer placed on an emissive polymer layer that emits the color blue can provide a lower drive voltage than a second electron injecting layer comprised of a different material. Specifically, in terms of overall performance, barium and calcium are the most common and effective materials for use as the electron injecting layer in OLED devices that emit the color green or yellow. For an OLED that emits the color blue, lithium fluoride is the most common material for use as the electron injecting layer in this type of OLED (the lithium fluoride is typically capped by a calcium layer). When fabricating full color displays, it is desirable to have one electron injecting layer that can be effectively used with emissive polymer layers that emit any of the colors.
For the foregoing reasons, there exists a need for an electron injecting layer that has a low work function, is less likely to react with the environment, can be easily deposited, and in the case of an organic light emitting diode, effectively interfaces with different emissive polymer layers that emit different colors.